The invention relates to a method permitting multiple uses of a copper wire which is used in a first application as a wire electrode for electrical resistance seam welding, in particular for tinned sheets. For this purpose, the wire is formed into a wire with a cross section departing from the circular form, particularly into a flat wire, before the first use, and is hardened in that form. The wire is contaminated during the first application in the region coming into contact with the weld by material originating from the weld and passing over the wire during welding.
From the earliest known research for electrical resistance seam welding of tinned sheet metal using copper wire as the wire electrode, it had been considered to use the copper wire employed as the electrode multiple times.
Such was already thought of, for example, in the first unproductive research described in U.S. Pat. No. 2,838,651 for electrical resistance seam welding of tinned sheet metal with the application of wire electrodes, the electrodes established contact between the electrode roll and the weld and thereby prevented contamination of the electrode roll by the tin from the weld. It was thought that the wire electrodes coming into contact with the weld and contaminated thereby due to the tin passing over the wire from the weld could be freed of this contamination and accordingly could be renewed to be used again as a wire electrode in the same welding machine (U.S. Pat. No. 2,838,651, column 4, lines 39-54). Here consideration was given to removing the contamination from the wire electrode in that the wire electrode was to be guided through a calibrated hole with a diameter slightly smaller than the original wire diameter "so that a thin outer coating of the wire is removed by the process and thereby the removal of any type of foreign material from the wire is guaranteed." (U.S. Pat. No. 2,838,651, column 4, lines 43-45).
Actually, however, the method described in U.S. Pat. No. 2,838,651 cannot be put into practice for a number of reasons. The principal reason for this is that the wire electrode is flattened in passing the welding point under the pressure of the electrode roll and is thereby lengthened. (The effect of this flattening of the wire electrode and the resulting wire lengthening is known, for example, from Swiss Pat. No. 436,512 and was not overcome for over ten years after the publication of U.S. Pat. No. 2,838,651 by the features described in the Swiss Pat. No. 536,163). Because of the wire lengthening resulting from the flattening under the first of the two electrode rolls, which the electrode wire has to pass according to the method of U.S. Pat. No. 2,838,651 and the condition that requires the wire electrode to pass the two electrode rolls at the same speed (U.S. Pat. No. 2,838,651, Column 4, Lines 9-33), the result is that a bend or bow occurs in the wire slide between the first and second electrode rolls which leads practically to a functional inability to accomplish the method according to U.S. Pat. No. 2,838,651. The effect of flattening and the bow formation resulting therefrom can be substantially overcome later, as is already mentioned above, by the features of Swiss Pat. No. 536,163.
Apart from the general functional inoperability of the method described in U.S. Pat. No. 2,838,651 resulting from the stated flattening of the electrode wires, the anticipated passage of the wire electrode through the calibrated orifice did not lead to the intended result. Because the flattening of the wire electrode with the pressure of the electrode rolls leads necessarily to the fact that those regions of the wire electrode which come into contact with the weld and are thereby contaminated by tin also immediately, are the same areas which are flattened and leveled off. If now, as stated above, a thin outer coating of the wire is removed by the die, then the flattened and leveled peripheral regions of the wire electrode will not be comprehended or included with this removal. This is because the distance of the flattened regions from the axis or correspondingly the wire center is less than the distances of the other peripheral regions of the wire after the removal of the thin outer coating, so that even in those regions of the wire which are contaminated with tin, the contamination will not be removed by the passage of the wire electrode through the calibrated hole. Besides, the general concept of making wire electrodes utilized in resistance seam welding usable again in the same welding machine after its use by means of a reduction in diameter is naturally in practice not entirely realized, because the wire electrode must be in adjacent relationship to the electrode roll with a relatively large outer surface so that the contact resistance between the electrode rolls of the wire is made as small as possible and so that a welding of the electrode wire to the electrode rolls is foreclosed. Such a relatively large contact surface between the wire electrode and the electrode roll is therefore only achievable if the electrode fits precisely in the groove provided for it in the electrode roll and with this its peripheral region lying within the groove fully adjacent the groove walls, and if the bending radius of the groove walls is the same as the radius of the electrode wire. These conditions are naturally no longer fulfilled with the diameter of the electrode wire reduced. In other words, the reduction of the diameter of the electrode wire arising from the calibrated orifice leads to the fact that the electrode wire, no longer flat shaped, but only linearly shaped, lies in the groove provided for it in the electrode roll and produces thereby a larger contact resistance between the electrode roll and the wire electrode and with it the danger of welding the electrode wire to the electrode roll. In addition, there is still the fact that with an electrode wire of reduced diameter there is simultaneously also a poor guidance of the wire in the groove provided in the electrode roll which can lead to a jumping of the wire from under the electrode roll in the welding mechanism.
For all of these reasons, the concept of a multiple use of the electrode wire in the same welding machine as expressed in U.S. Pat. No. 2,838,651 was not in practice realized and no suggestions to one of ordinary skill in the art could bring about such a multiple usage.
Also again in the later Swiss Pat. No. 370,175, multiple uses of electrode wires were considered in a welding machine serving for electrical resistance seam welding of coated sheet metal, and these also proved not to be practical. To be sure, there was no reduction in the diameter, only cleaning of the wire electrode after each passage so that therefore the problems existing with the reduction of the diameter of the wire electrode mentioned above in conjunction with the process of U.S. Pat. No. 2,838,651 did not appear. However, this cleaning also did not lead to the desired result that such already used and cleaned wire electrodes would lead to welding results approaching those equally as good as with a new wire. The reasons for this lie not only in the cleaning method employed but were in large measure system related, that is, in principle, inherent. With the cleaning methods employed, the principal method considered, i.e., the pulling of the electrodes wires through a cleaning die after use, with the impurities adhering to the wire in the form of tin parts being stripped away (Swiss Pat. No. 370,175, page 2, lines 27-30) was already considered to be unsatisfactory because of this. The adhering tin naturally did not permit itself to be stripped away in the cold condition and therefore only a very incomplete removal of the tin from the outer surface of the wire was made possible by means of this cleaning die which could not in any way lend to the wire an outer surface quality that was comparable with a new wire. Also, a further chemical cleaning method consisting of dissolving the tin from the outer surface of the wire by means of a diluted hydrochloric acid or a diluted caustic soda solution (Swiss Patent No. 370,175, Page 2, Lines 35-38) was practically then not considered feasible because such a chemical solution would have lasted too long and besides the hydrochloric acid or correspondingly the caustic soda would have attacked the outer surface regions of the wire covered with tin not at all or very weakly, and with that would have led to an unacceptable danger of wire breakage at heavily attacked wire positions. In contrast, a method of cleaning was likewise considered by melting tin from the used wire electrodes (Swiss Pat. No. 370,175, page 2, lines 38-41), for example, by passing the used wire electrode through a tin bath as well as a die positioned at the output of the bath to strip the fluid tin. The method at first appeared to be very promising because it permitted an outer surface quality of the used wire to be achieved that was at least as good as a new coated wire.
As it turned out, however, this form of cleaning also did not lead to a wire electrode that was usable again with welding results that were as good as those achieved with an (uncoated) new wire. The reason for this lay in the fact that the tin passing over the wire electrode during welding not only adhered to the outer surface of the copper wire but also penetrated the alloy formations in the copper coating lying on the outer surface, and the copper-zinc alloy created thereby in the outer surface coating of the wire had a specific resistance which amounted to a multiple of the specific resistance of the copper and was even substantially above the specific resistance of tin which itself is already approximately 6.5 times the specific resistance of copper. As a result of this high specific resistance of the copper-tin alloy stemming from welding by the wire electrodes in the outer surface coating of the wire during reuse of the wire, an extraordinarily intense heating is produced in this outer surface coating with the flow of welding current. This can lead to fusing of the wire to the weld, and in each case, to a substantially poorer weld quality than with the use of a new wire. Subsequent research showed then also that this reason for not reusing the wire was of decisive import, not only with cleaning by melting the tin but also with the other mentioned cleaning methods. It also showed thereto that by careful mechanical or chemical removal of the tin adhering to the outer surface of a used wire electrode as a result of the copper-tin alloy in the outer surface coating of the wire does not yield good welding results with the reuse of such a carefully cleaned wire electrode and that accordingly the formation of the copper-tin alloy coating in the outer surface coating of a wire electrode during welding is a fundamental impediment to a reuse of an already used wire electrode. A further basic impediment of a principal nature which stood in the way of the reuse of an already used and cleaned wire electrode as considered in the Swiss Patent No. 370,175 was also the previously mentioned flattening of the wire electrode under the pressure of the electrode rolls. This flattening naturally leads with each repeated use to a new and different cross section of the wire electrode and to achieve a constant weld quality, constant relationships, including a constant cross sectional shape of the wire electrode are indispensable quite apart from the fact that such variations of the cross sectional shape also required a change of the die employed for cleaning after each process to accommodate the altered cross section of the wire.
For the aforementioned reasons, the form of multiple uses of a wire electrode considered in Swiss Patent No. 370,175 were not practically obtainable in the same welding machine.
From the beginning of the practical realization of welding machines for electrical resistance seam welding of coated sheet metal with the help of wire electrodes, it appeared that a multiple use of the wire electrode was not possible. This was primarily due to the referenced fundamental impediments of the formation of a copper-tin alloy in the outer surface coating of the wire electrode with welding and the flattening of the wire electrode under the pressure of the electrode roll. No other choice remained for avoiding a loss of the relatively expensive copper employed for the wire electrodes than to melt the contaminated wires after their singular use. This choice corresponds to the suggestion already given in Swiss Pat. No. 370,175, page 2, lines 20-21. This process sets the melted, contaminated copper free from the tin in a refining process, the tin having been brought in to the process with the contaminated wires, partly in the form of a copper-tin alloy and partly in the form of tin adhering to the wire.
In the first years of practical application of the welding machine for electrical resistance seam welding of plated sheet metal with the help of wire electrodes, the gathering of the contaminated electrode wires after their singular use resulted in the used wire electrodes being wound onto a spool, which, for example, is apparent in Swiss Pat. No. 436,513, (FIG. Part 44). This method of gathering the used wire electrodes was, however, not advantageous for the anticipated reclaiming the copper insofar as the removal of the wire from the spool in each case required special handling, namely, either a time-consuming unwinding of the wire from the spool, or on the other hand, the use of special collapsible spools which permitted a removal of the entire wire winding from the spool. After a certain time, this problem was overcome and the used wires were no longer wound a spool as is apparent, for example, from Swiss Pat. No. 519,961 (page 2, line 14, and FIG. 1). There, the used wire was stored in so-called container windings as are known, for example, from Swiss Pat. No. 552,336 (page 1, lines 65-69, FIG. 1, part 37). In the meantime the effective welding speeds had also increased substantially in comparison to those which had previously been achieved with the resistance seam welding described in Swiss Patent No. 436,513. With this increase of the effective welding speed, naturally also the assault on the used wire per unit of time was increased substantially. Also correspondingly enlarged container windings were employed, which enabled the daily yield of used wire to be assimilated. In the meantime, further development included in particular overcoming the already mentioned bow formation with resistance seam welding with only one wire electrode running over the two electrode rolls, as described in Swiss Pat. No. 536,163. This was accomplished by using a wire hardened by flattening before the introduction of the wire into the first electrode roll. With this further development, the effective welding speeds were sharply increased in comparison to those which had previously been produced by the method of welding described in Swiss Patent No. 436,513, which had only one wire electrode running over to the electrode rolls. In this method, this wire was wound onto a roll after use. Since this increase in the effective welding speed naturally also increased substantially the attack on the wire used per unit of time, correspondingly large container windings were also employed, which windings could accept the quantity of used wire attacked daily. Here then in conjunction with the above-mentioned melting and refining began the problem that the incoming volume from the irregularly stored wire in the container winding was substantially larger than the proper copper volume of the wire and yielded with it also an incomplete utilization of the capacity of the copper converter for refining. In order to avoid this insufficient utilization of the copper converter and for better utilization of the same to achieve the largest possible ratio of proper copper volume to that incoming volume from the used wire, the process was changed to one in which the used wire was cut into short wire pieces of approximately 20 to 40 millimeters in length right at the welding machine by means of the so-called wire cutters. This was apparent, for example, as indicated in the Soudronic Prospectus "VSA 50 E" (second page, righthand column, second paragraph), and also from the Schuler-Prospectus "LCS Body Welder" (page 7, diagram wire guidance and righthand column, lines 1-4). Accordingly these "wire clippings" held in a container were then guided into a copper converter for reclaiming the copper of the process. The copper pieces naturally required a substantially smaller volume in the container than a corresponding quantity of the wire irregularly stored in the said winding container.
A further development of this technique applied practically in the last ten years, in which the electrode wire is cut into short pieces after a single utilization and from these then the copper is reclaimed by means of melting and refining, represents the previously unpublished proposal, which was performed in recent times. This proposal comprises cutting the wire into substantially finer wire pieces of only 1-3 millimeters in length and placing these "wire granules" with the help of a press under such a high pressure that the copper is transformed into a fluid condition, in some cases with additional warming. Then the copper can be pressed in accordance with a type of well known extrusion presses through a matrix into a copper strand of 2-5 millimeters diameter which afterward can be drawn in a drawing device into a wire with a suitable diameter to again be employed as a wire electrode. With testing for technical achievement of this proposal, however, a series of unforeseen problems arose, in particular with the transformation of the granules into a thick, flowable material without captured air. These problems caused considerable difficulties and until now could only be overcome with experimental models. In each case, this process was successful for a long time only in the experimental arena to produce wires which were again insertable as wire electrodes in welding machines. Quite apart from this, it has been proven that the technical expenditure for this production method is substantially higher on account of the original problems and the resulting high investment costs for a design corresponding to the proposal than if one melted the referenced wire pieces in a conventional manner, refined these, extruded them, and then drew the wires from these extrusions.
In summary, it can be said that with all the known and proposed forms of recycling electrode wires used and consumed in electrical resistance seam welding with wire electrodes, it is commonly held for a long time that in practice only single uses of copper wires forming such electrodes are possible. Only the copper is capable of being covered from the single-use wire electrodes and all experiments and proposals for a multiple utilization of these copper wires have been frustrated in practice until now.
The invention has as a principal object to provide a method of the foregoing type by which the copper wire can in practice actually be passed on to a further utilization after its first use as a wire electrode for electrical resistance seam welding. Nothing else remains any longer than to introduce the used electrode wires of waste value for reclaiming of the copper.
In accordance with the invention, it is possible to accomplish the method of the above-mentioned type by passing the formed and contaminated copper wire after its first use to a wire drawing device consisting of a plurality of drawing steps, and there to draw the wire in such a manner that the distances of the peripheral regions of the wire most distant from the center of the wire are reduced in all drawing stages and the distances of the peripheral regions of the wire lying nearest to the center of the wire are reduced in at least part of the drawing stages. Further, the distances of the peripheral regions of the wire most distant from the center of the wire are reduced successively in the drawing stages at least to such an extent that they become smaller than the distances of the peripheral regions of the wire nearest to the center of the wire before drawing, and the distances of the peripheral regions lying nearest to the center of the wire are reduced in said part of the drawing stages to such an extent that after passing the last drawing stage, they are as large as the distances of the peripheral regions of the wire having previously been most distant from the center of the wire. In this manner, a new round copper wire with circular cross section and a smaller diameter than the smallest outer dimension of said formed and contaminated copper wire is drawn from said formed and contaminated copper wire in the wire drawing device, and this new round copper wire is supplied for a further application.
The principal advantage of the foregoing method is that for the first time resistance seam welding of plated sheet metal is made possible with the help of wire electrodes whereby the wire electrodes no longer simply add to the operating costs, but to the contrary, contribute to a reduction of the ordinary operating cost because the new copper wire stemming from the foregoing method has a smaller diameter than the electrode wire originally employed. For this reason, the value of the new round copper wire stemming from the foregoing process per unit weight is greater than the value of the copper wire originally employed as the wire electrode. Surprisingly, it can be established that the properties of the new copper wire derived from the foregoing process differ not at all or only in a negligible degree from the properties of a corresponding wire drawn directly down to the same diameter in a wire drawing process and lie well within the standard tolerances. Therefore, contrary to every expectation, the tin from the wire electrodes after use in resistance seam welding that is carried with the plated sheet metal, partly adhering to its outer surface and partly contained in the copper-tin alloy in its outer surface coating, influences the properties of the round copper wire drawn from the wire electrodes in a negligible manner. This is just as true for the electrical properties, for example, the specific resistance, as well as for the mechanical properties, for example, the hardness and ultimately as well as for the appearance of the newly drawn round copper wire and the shiny gap-free outer surface of the same. Further research on these surprising results suggests that the tin carried along from the wire electrodes obviously shifts while drawing the wire into the regions of the so-called slip planes where various zones of the copper crystals substantially forming the wire rub against one another during drawing. This is the reason that, for example, the electrical conductivity of the wire in its longitudinal direction and also the tensile strength of the wire in the longitudinal direction for all intents and purposes is not influenced at all by the injected tin. The original tin adhering to the electrode wire is obviously deposited in the form of an atom like coating on the exterior surface of the wire as long as it does not get into the wire interior in the drawing process, and accordingly cannot influence the electrical and mechanical properties of the wire in its lengthwise direction in any case. In this connection, it is appropriate to note that in resistance seam welding by means of wire electrodes the welding current itself does not flow in the longitudinal direction of the wire but in a transverse direction through the wire. It is therefore easy to explain why already used wire electrodes provide substantially poor welding results upon reuse because there, of course, the welding current must flow through the exterior copper-tin alloy and the tin adhering to the wire surface, while a current flowing in the lengthwise direction of the wire flows past such outer surface coatings.
In each case, with the foregoing method the results produced for the first time since the original development of welding machines with electrical resistance seam welding showed that the electrodes employed for welding did not give rise to perpetual operating costs but instead contributed to a reduction in the ordinary operating costs. This is because, as earlier, so still if goods were welded directly with electrode rolls (and the welding of coated sheet metal which was only first made possible through the application of wire electrodes was still not possible), the operative surfaces of the electrodes coming in contact with the welded goods (usually shiny steel sheet metal) were consumed. After a certain operating time, this led to an inferior weld quality so that the electrode rolls had to be changed from time to time after a particular operating time and with this naturally came perpetual operating costs.
With the drop in perpetual operating costs for the electrodes, the foregoing method of resistance seam welding accordingly brings forth generally a substantial technical advance. With it within the realm of resistance seam welding of shiny steel sheet metal, for which welding directly with electrode rolls was not outweighed on grounds of cost until now, one can proceed to welding by means of wire electrodes and thereby produce savings in addition to the improvement of the weld quality.
Further advantages can also be achieved in the realm of resistance seam welding with the foregoing method and the help of wire electrodes. So, for example, with the application of the foregoing process resulting in a drop-off of the perpetual operating costs for the electrodes resulting from it, it is possible without more to proceed from the method of welding with only one wire electrode guided over an electrode roll. This welding with one wire electrode was employed almost exclusively in the last two decades in the technique of resistance seam welding with the help of wire electrodes back to the original form of welding known, for example, from Swiss Patent No. 370,175 with a special wire electrode for each of the two electrode rolls. With the new method, it is possible to simplify the entire process of resistance seam welding with the help of wire electrodes without again incurring higher operating costs, because the conversion from welding with just one special electrode for each of the two wire electrodes on the one hand to welding with only one wire electrode guided over the two electrode rolls was certainly exclusively for the reduction of wire consumption. The operating costs associated with the conversion were taken in hand but brought with it a few difficult technical problems which could be overcome only with an increased apparatus expense for the corresponding welding machine. The application of the foregoing process and the simultaneous conversion to welding with just one special wire electrode for each of the electrode rolls permitted this increased apparatus expense to be again reduced whereby simultaneously the operating costs still dropped off. These costs until now had still been incurred for the wire electrodes guided over the two rolls.
Additional advantages are achievable with a further embodiment of the invention.
As a rule, the new copper wire drawn in the drawing device with the foregoing method is guided or passed from one of said first uses to a further, different use. One of the first uses being identical to the further use is not generally foreclosed, however, it is scarcely ever considered in practice because the diameters of the new drawn wire as a rule lie below the ordinary range of diameters for wire electrodes employed for resistance seam welding. The further use differing from said first use can advantageously be one such that the new copper wire is employed for conducting current in the direction of the wire and, if appropriate, in a permanent application, preferably as a signal conduit or wire winding. The advantage of one of said first uses differing from the further use is that in this case as is already mentioned above, the effects of the properties of the new drawn copper wire are foreclosed by the first utilization.
Advantageously, the new copper wire drawn in the drawing device by the foregoing method can be provided with an insulating sleeve, preferably with a coating of paint, before passing the wire further on to said additional application. This is true in particular in the case of said application as a signal conductor or wire winding. Also, it can be advantageous for these and other applications to anneal the new drawn copper wire in the drawing machine after passing through the last stages by heating the wire to a temperature corresponding at least to its recrystallization temperature and preferably to a temperature lying above this temperature.
If the copper wire is hardened so much by the foregoing process before its first use that its elongation after the first utilization is still less than ten percent, the deformed and contaminated copper wire after its first utilization should be annealed appropriately before passing it through the first stage of the drawing device. This is accomplished by heating the wire to a temperature corresponding at least to its recrystallization temperature, and preferably to a temperature lying above this temperature. The annealing has the advantage that wire breakage as a result of working the wire during processing through the drawing device is substantially foreclosed.
If a wire drawing device employed in the foregoing process has an operating capacity per unit of time which amounts to a multiple of the consumption rate of the electrodes per unit of time from one of said electrical welding machines employed for the resistance seam welding, then it is substantial advantage for improved use of the operating capacity of the drawing machine to gather the wire electrodes after their use as wire electrodes from a plurality of said welding machines used for seam welding, and to feed these gathered, deformed, and contaminated copper wires one after the other to the drawing machine. For gathering of a wire electrode after its use in a welding machine, a driven spool can be employed appropriately, preferably a flanged spool onto which the electrode wire is brought in layers. This operation preferably is controlled by the welding machine and the spool is preferably located on the welding machine. It is advantageous to employ a common drawing device for a plurality of welding machines corresponding to those with the attacked and used wires, in particular for large production facilities with a plurality of welding machines. There, a separate drawing machine is worthwhile and such special machines designed for wire drawing can process substantially larger quantities or rates of wire than are attacked by one welding machine. One such unassociated wire drawing machine, however, naturally can process with advantage the used wires gathered from a plurality of smaller production facilities having only one or two such welding machines.